In the related art, in the actuator, the ultrasonic device such as the ultrasonic oscillator, the ultrasonic motor, the pressure sensor, the pyroelectric element such as the IR sensor, and the like, high piezoelectric properties are demanded for a piezoelectric material that is used as a piezoelectric layer (piezoelectric ceramic) constituting a piezoelectric element or the like which is mounted on a device (piezoelectric element application device) using a piezoelectric element as a drive source. Examples of properties which are demanded for a piezoelectric substance includes various properties such as a piezoelectric constant, a dielectric constant, and Young's modulus, and the invention gives attention to a piezoelectric constant (d33). Hereinafter, the “piezoelectric properties” represent the piezoelectric constant (d33). Representative examples of the piezoelectric material having high piezoelectric properties include lead zirconate titanate (PZT).
However, there is a demand for a piezoelectric material in which an amount of lead that is contained is suppressed from the viewpoint of an environmental problem. Example of a lead-free based piezoelectric material include a piezoelectric material such as KxNa(1-x)NbO3 and (Ba, Na)TiO3 which contain an alkali metal, and a piezoelectric material such as BiFeO3—BaTiO3 that does not contain the alkali metal.
In the piezoelectric materials, there is known that when using a composition in the vicinity of a morphotropic phase boundary (MPB), large piezoelectric properties are obtained. However, with regard to a phase diagram in which the horizontal axis represents a composition, and the vertical axis represents a temperature (hereinafter, referred to as a phase diagram), in PZT, an MPB line is located to be approximately parallel to the temperature axis or approximately perpendicular to the composition axis. In contrast, in the lead-free-system piezoelectric material, generally, the MPB line is inclined with respect to the temperature axis (For example, refer to FIG. 1 and the like of Japanese Unexamined Patent Application Publication No. 2009-215111). In a case where the MPB line is inclined as described above, even when selecting a specific temperature in accordance with desired properties, for example, a composition that is located on the MPB at room temperature, there is a problem that a temperature region, in which the piezoelectric properties decrease due to a variation in the use environment temperature, heat generation during use, and the like, exists when considering that if the use environment temperature varies, the composition is distant from the MPB line.
Accordingly, there is a demand for a lead-free-system piezoelectric material in which the MPB line is parallel to the temperature axis as much as possible in the above-described phase diagram, and temperature dependence of piezoelectric properties is small. In addition, although it is needless to say that high piezoelectric properties are demanded for the piezoelectric material, a high Curie temperature (Tc) is an important condition that is demanded for the piezoelectric material from a relationship with the use environment temperature. This is because the piezoelectric material functions only at a temperature lower than the Curie temperature. A piezoelectric material in which the Curie temperature is high can be used in a relatively wide temperature range, and thus general-purpose properties are high. However, generally, the piezoelectric material in which the Curie temperature is high has a tendency in which piezoelectric properties are poor.
Here, it is considered that a plurality of piezoelectric materials having compositions different from each other are used in combination so as to obtain a piezoelectric material satisfying conditions such as making the MPB line parallel to the temperature axis as much as possible, and reduction in temperature dependence of piezoelectric properties (refer to Japanese Unexamined Patent Application Publication No. 2003-277143, Japanese Unexamined Patent Application Publication No. 2011-181764, and the like). In addition, in the following description, in a phase diagram in which the horizontal axis represents a composition and the vertical axis represents a temperature, a state in which the MPB line is nearly parallel to the temperature axis, or a state in which the MPB line is nearly perpendicular to the composition axis is expressed by “MPB line stands up”.
However, there is no still finding indicating a clear index regarding to whether or not a piezoelectric material, in which the MPB line stands up and thus the temperature dependence of piezoelectric properties is less, can be obtained in what kind of combination of what kinds of components, whether or not a piezoelectric material, in which the piezoelectric properties are high, can be obtained in what kind of combination of what kinds of components, and whether or not a piezoelectric material, in which the Curie temperature is high, can be obtained in what kind of combination of what kinds of components. In addition, among a plurality of conditions such as less temperature dependence of piezoelectric properties, high piezoelectric properties, and a high Curie temperature, when only one condition is satisfied, it cannot be said that the material is a piezoelectric material with high practical utility. It is preferable that a composition simultaneously satisfies the plurality of conditions so as to obtain a piezoelectric material with high practical utility. There is no clear index, and thus it is very difficult to find a combination and a composition of components as described above. Currently, as the piezoelectric material that simultaneously satisfies the plurality of conditions as described above, substantially, only PZT can be exemplified. In addition, a lead-free-system piezoelectric material comparable to PZT is not present. Accordingly, there is a demand for the lead-free-system piezoelectric material in which the MPB line stands up, the piezoelectric properties are high in a wide operating ambient temperature, and the Curie temperature is high similar to PZT.
In consideration of the above-described situations, an object of the invention is to provide a piezoelectric material in which an environmental load is low and which is excellent in practical utility, a method of manufacturing the same, and a piezoelectric element and a piezoelectric element application device which use the piezoelectric material.